The present invention pertains to a method and apparatus for cleaning screens which are used for filtering liquids, e.g., paints, adhesives and coatings. More particularly, it pertains to a novel spray system comprising spray bars with multiple nozzles located in close proximity to a coating filter screen for backwashing the screen and for removing particulates collected thereon.
Filter screens are used in many different industries for removing particulates, debris and other foreign matter from the products being manufactured or from products used in the manufacturing process. In the present invention, the screen comprises a part of a paper coating process and is used to filter the coating materials which are applied to the paper to make the paper glossy and smooth.
Coating screens of the type disclosed herein are located between the coating kitchen where the paper coatings are made up, and the coating pan where the coatings are actually applied to the paper. The coatings, as made up in the coating kitchen, are generally fairly clean, but they still must be screened to remove particulates greater than about 50-75 microns in size, to ensure good coating flow during the coating application process, and to eliminate the possibility of scratches on the paper surface. However, the coating material may become contaminated during the coating process when loose fibers from the web, which are dislodged during the coating process, end up as part of the recirculated coating material returned to the coating pan. These contaminates plus dried chunks of coating which flake off the coating pan, and dried coating layers which form within the screening chamber are also returned as make-up to the coating supply tank. Thus, coating screens are used in the coating kitchen where coatings are initially made up before the coating material is pumped to the coating supply tank, and for the purpose of cleaning make-up coating returned from the coating pan to the coating supply tank. During these filtering processes, the screens become clogged with contaminates and must be cleaned on a regular basis.
In the past, attempts by others to wash and clean coating screens have involved a number of different schemes. It is known, for instance to tilt a filter screen as it is being spray washed to remove contaminates. Other methods employed in the past for cleaning filter screens and tanks include spray nozzles that are movable to provide a variable spray area, nozzles on header pipes which travel over the screen, and nozzles on header pipes which are rotated above the filter screen to produce a cleaning action (see U.S. Pat. Nos. 3,587,975; 4,076,033; and 5,110,366). However, each of these cleaning systems have been found to include drawbacks and deficiencies, particularly for cleaning coating screens as disclosed herein.
An example of a typical coating screen used in the paper industry is the SWECO screen manufactured by the SWECO Mfg. Company, of Florence, Ky. 41022. A typical SWECO screen for paper coatings involves the use of one or more screens positioned horizontally in a screening chamber. The coating material is introduced at one end of the screening chamber where it spreads over the screen. Generally the screen or screens in the screening chamber are gently vibrated so that the filtered coating material falls by gravity through the screen into the bottom of the screening chamber where it can be collected and pumped to the coating apparatus. Large particulates and other debris and contaminates collect on the surfaces of the screens which must then be cleaned periodically to prevent the screen from becoming clogged.
The typical system employed in the past to clean such screens has involved the use of one or more cluster spray heads mounted above the screen. These spray heads may have up to thirteen (13) spray nozzles oriented in different directions. The function of the sprays from the nozzles are to wash down the interior surfaces of the screening chamber as well as to remove any contaminates accumulated on the screen surface. However, screen cleaning methods of this type have proven to be ineffective since the sprays apply water to the surface of the screen haphazardly and in a completely random manner while removing little if any of the accumulated debris. Also, the conventional cluster nozzles have been found to fail to adequately clean the interior surfaces of the screening chamber, and over time, large chunks of dried coating which form on the screening chamber walls become dislodged and fall on the screen surface causing damage to the screens. Accordingly it will be seen that there is a need for an improved means for periodically cleaning the screens and screening chamber interior of the typical coating screens used in the paper industry.